Coating composition, preferably printing ink for security applications, method for producing a coating composition and use of glass ceramics

ABSTRACT

The present invention relates to coating compositions, preferably printing inks for security applications comprising at least one organic resin, at least one pigment and optionally at least one organic solvent. Said pigment comprises glass ceramic composite particles, containing at least one crystalline particle embedded in a glass matrix. Said glass ceramic particles have a particle size in the range of between 0.1 μm to 50 μm. Preferably active ions selected from the group of the rare-earth elements are incorporated into the crystalline phase of the composite to provide the glass ceramics with luminescent up- and down-converting characteristics. Glass ceramic luminescent have excellent physical and chemical stability. The glass matrix permits as well the stabilization of the photophysically interesting halide host crystals which have low phonon energies. Such materials provide unusual excitation and emission properties.

The present invention relates to a coating composition, preferably aprinting ink for security applications, to a method for producing acoating composition and to the use of glass ceramics according to thepreambles of the independent claims.

Pigments which have luminescent properties (phosphors) are well knownand are widely used as marking materials in security applications.Luminescent materials can absorb certain types of energy acting uponthem and subsequently emit this absorbed energy as electromagneticradiation. Down-converting luminescent materials absorb electromagneticradiation at a higher frequency (shorter wavelength) and re-emit it at alower frequency (longer wavelength). Up-converting luminescent materialsabsorb electromagnetic radiation at a lower frequency and re-emit partof it at a higher frequency. Luminescent materials are used for codingand marking of mass-produced goods, high value branded articles andsecurity documents. In certain cases an up-converting luminescent isadded as a hidden “taggant” to a transparent or colorless coatingcomposition or printing ink, which is applied to branded goods in formof barcodes, company emblems, labels, etc. This allows a subsequentrecognition of the genuine article in the fight against counterfeitersand product piracy.

Light emission of luminescent materials arises from excited states inatoms or molecules. The radiative decay of such excited states has acharacteristic decay time, which depends on the material and can rangefrom 10⁻⁹ seconds up to various hours. Short-lived luminescent emissionis usually called fluorescence, whereas long-lived emission is calledphosphorescence. Materials of either type of emission are suitable forthe realisation of machine-readable codes. Machine-readability is anecessary prerequisite for mass treatment of goods, e.g. in automatedproduction, sorting, quality control, packaging or authenticationoperations. Machine-verification is also applied outside production orlogistic chains for counterfeit or fraud detection.

The common up-converting materials are of inorganic nature and consistessentially of a crystal lattice in which rare-earth ions are present asactivators and sensitizers. The excitation and emission characteristicsof up-converting materials are inherent characteristics of the rareearth ions employed. Their corresponding optical absorption and emissionprocesses are due to electron transitions within the incompletely filled4f shell of the rare earth ion. This electron shell is strongly shieldedfrom the chemical environment of the atom, such that variations in thecrystal lattice, thermal vibrations, etc. have only a marginal influenceon it. Consequently, rare-earth ions have narrow band optical absorptionand emission spectra, which are to a great extent independent of thenature of the crystal lattice. The sharp, discrete bands and the lowinteraction with the crystal lattice usually result in a high saturationof the luminescence color and a high luminescence quantum yield.

Rare-earth ion luminescence activators have relatively long-livedexcited states and a particular electronic structure. This permits theenergy of two or more photons in succession to be transmitted to onesingle luminescence centre and cumulated there. An electron is thuspromoted to a higher energy level than that corresponding to theincoming photon energy. When this electron returns from its higher levelto the ground state, a photon having about the sum of the energies ofthe cumulated exciting photons is emitted. In this way it is possible toconvert e.g. IR radiation into visible light. Alkali and alkaline earthmetal halides, and the halides, oxyhalides and oxysulfides of yttrium,lanthanum and gadolinium are principally used as the host material,whereas e.g. Er³⁺, Ho³⁺ and Tm³⁺ serve as the activators. Additionally,ytterbium(3+) and/or other ions can be present in the crystal lattice assensitizers to increase the quantum yield.

Down-converting luminescents are either of inorganic or of organic(molecular) nature. Irradiation of the luminescent with short-wave lightpromotes an electron to a higher excited state. Decay of this higherexcited state usually follows a cascade to next-lower excited states,and finally to the ground state, and produces light emissions havinglonger wavelength than the exciting radiation. Typical down-convertingluminescents convert UV to visible light. Conversion of UV or visiblelight to IR, or of lower wavelength IR to higher wavelength IR is alsopossible. Usually up-converting luminescents can also be exploited indown-converting modes.

However a lot of up-and down-converting materials are not stable whenexposed to oxygen, humidity, and, in particular, to organic solventsand/or media containing chemical oxidising or reducing agents. Thus thechoice of luminescent materials, particularly of up-converters which aresuitable for being blended as pigments into polymer compositions, suchas coating composition or printing inks, is limited to only a few typesof host crystals.

GB 2 258 659 and GB 2 258 660 describe up-converting materials based onyttrium oxysulfide (Y₂O₂S), doped with erbium and ytterbium. Furtherdisclosed is the use of such materials as pigments in printing inks forsecurity applications.

Since compositions, synthesis and absorption/emission properties of thecommon up- and down-converting materials fulfilling the necessarystability criteria are more and more known to counterfeiters as well,there is a constant need for new up-and down-converting materials,having uncommon composition and properties, such as particularluminescence decay characteristics, and/or particular luminescenceefficiency and/or, in its case, particular branching ratios betweenmultiple emission possibilities, all of them being exploitable forsecurity purposes.

It is an object of the present invention to overcome the drawbacks ofthe prior art.

Particularly it is an object of the invention to provide new luminescentpigments, especially those having unusual excitation/emissioncharacteristics. It is a further object of the invention to provide up-and down-converting pigments which are resistant to environmentalinfluences, particularly against organic resins and/or solvents.

These objects are solved by the features of the independent claims.Particularly they are solved by a coating composition, preferably aprinting ink for security applications, comprising at least one organicresin, at least one pigment and optionally at least one solvent,characterised in that said pigment comprises glass ceramic particleswhich contain at least one crystalline phase embedded in a glass matrix,said pigment having a particle size in the range of between 0.1 μm to 50μm.

Preferably the glass ceramic particles have a particle size in range ofbetween 1 μm to 20 μm and even more preferably in the range of between 3μm to 10 μm.

Glass ceramics are composite solids, which are formed by controlleddevitrification of glasses. (See Römpp Chemie Lexikon, ed. J. Felbe, M.Regitz, 9^(th) edition 1990, page 156.) They can be manufactured byheating (tempering) suitable precursor glasses to allow for partialcrystallisation of part of the glass composition. Glass ceramicscomprise thus a certain amount of a crystalline phase, embedded in asurrounding glass phase.

In a preferred embodiment of the present invention the crystalline phaseof the glass ceramics comprise a luminescent material. This is ofparticular interest and value for luminescent materials, which are notstable in an ordinary environment, and which can in this way beprotected from the adverse influence of oxygen, humidity, etc. The glassmatrix protects the crystalline phase from dissolution in an adverseenvironment, and permits incorporation into a coating composition or thelike. New types of luminescent materials are thus amenable to printingapplications by this method.

Many photophysically interesting luminescent host materials are e.g.water soluble to a certain extent, like the fluorides, chlorides orbromides of the lanthanide elements. The solubility is due to the ratherweak electrostatic crystal lattice forces tied to mono-negative anions.The same materials show, due to the same reason and/or to the presenceof heavy ions, only low-frequency vibrational modes (phonon modes) oftheir crystal lattices. The absence of high-frequency vibrational modesresults in largely increased excited state life times and luminescencequantum yields. The reason for this is that the probability ofvibrational desexcitation of an electronically excited activator ion islow if the energy gap to the next lower lying electronic level is muchlarger than the energy of the highest vibrational mode (phonon energy)of the crystal lattice. Energy transfer to the crystal lattice becomesnegligible in such cases. Host materials with low phonon energy wouldthus be highly desirable, especially in the field of up-convertingphosphors where long-lived excited states are needed for achieving highquantum yields. The water-solubility and moisture sensitivity oflanthanide halides and related materials has up to now preventedcorresponding technical applications.

Preferably the crystalline component of the glass ceramics has a phononenergy not exceeding 580 cm⁻¹, preferably not exceeding 400 cm⁻¹ andeven more preferably not exceeding 350 cm⁻¹. These values stand forrather low-phonon energy, which are especially suitable as luminescencehosts because they allow for emissions from excited energy levels thatwould otherwise be quenched in high phonon energy solids, such as oxidesor the like.

Phonons, as mentioned, are crystal lattice vibrations in a material. Therelevant phonon energy is tied by Planck's relationship E=hv to thefrequency v of the highest measured MIR absorption band of the compound.If an excited rare earth ion has a transition possibility between twoenergy levels of interest, that corresponds to only a few times thephonon energy of the host lattice, the energy will be preferably andrapidly dissipated to the crystal lattice, without emission ofelectromagnetic radiation (radiationless transition). In a host latticewith much lower phonon energy, the same transition will preferablyradiate. In intermediate cases, both processes, radiating, andradiationless desactivation, will compete with each other.

In the Pr³⁺ ion, the ¹G₄ level of Pr³⁺ is only 3000 cm⁻¹ above the ³F₄level. In an oxide matrix, such as a praseodynium glass, only few Si—Ovibration phonons (1100 cm⁻¹) are required to bridge this gap. Thus anyexcited electron in the ¹G₄ level will rapidly return to the ³F₄ levelby exciting crystal lattice phonons, and no electromagnetic radiation ofthe corresponding wavelength is produced. In a Pr³⁺ doped LaF₃ matrix,the phonon energy is 350 cm⁻¹, and the ¹G₄ to ³F₄ transition of the Pr³⁺ion occurs radiatively. Additionally, the live time of the ¹G₄ state isstrongly increased.

Since phonon energies are controlled by the bond strengths and themasses of the ions forming the crystal lattice, heavy elements with weakbonding will provide the lowest phonon energy materials. The heavy metalfluoride glasses such as e.g. ZBLAN (53ZrF₄•20BaF₂•4LaF₃•3AlF₃•20NaF)have half the maximum phonon energy of silicates and thus take twice asmany phonons to quench the ¹G₄ level of Pr³⁺. ZBLAN glasses, a wellknown host lattice for laser and fibre-optic applications, can also beused as the glass component of glass ceramic composites according to thepresent invention.

Preferably the glass ceramic is substantially transparent toelectromagnetic radiation in the range of between 400 nm to 750 nm, i.e.in the visible range of the electromagnetic spectrum. Transparency ofglass ceramics are determinated by the average dimensions of theembedded crystals and/or the refractive index difference between thecrystals and the glass matrix.

In a preferred embodiment average dimensions of the crystals are notexceeding 50 nm, preferably not exceeding 40 nm. Exceeding crystal sizeresults in opaqueness of the glass ceramic.

Preferably, the average distance from one embedded crystal to another inthe glass matrix should be in the order of the crystal size, e.g. notexceeding 50 nm and preferably not exceeding 40 nm. Apart fromtransparency another important aspect is the protection of the crystalsby the glass matrix. Those host crystals having poor stability towardsenvironmental influences and being neither physically nor chemicallyresistant towards organic resins, solvents, humidity, etc. canefficiently be protected by a glass matrix having such chemical andphysical resistance. If the size of the embedded crystals are accordingto the preferred embodiment of the present invention, the grinding ofthe glass ceramics to pigment size particles is surprisingly possiblewithout adversely affecting the luminescent properties of the glassceramics. The photoactive crystals remain thus protected by thesurrounding glass matrix.

In a preferred embodiment at least one crystal in the glass matrixcomprises an active ion.

In the context of the present invention the active ion being present inat least one of the crystals in the glass matrix are rare earth ionshaving an appropriate electronic structure, particularly suitable arerare earth ions selected from the group consisting of Pr³⁺, Nd³⁺, Sm³⁺,Eu³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺ and Yb³⁺.

In a preferred embodiment of the present invention the glass ceramics isan oxyfluoride glass ceramics. Oxyfluorides have the low phonon energyof a fluoride matrix and the durability and mechanical properties of anoxide glass. The oxide glass will determine the mechanical and physicalproperties of the composite whereas the optical properties of the activeion will be controlled by the embedded fluoride crystalline phase.

A preferred glass matrix in the present invention for oxyfluoridesconsists essentially of NAS glass (Na₂O•Al₂O₃•SiO₂) NAS as host glassshows favourable properties with respect to melting and forming, goodtransparency and excellent durability. The content of SiO₂ preferably isbetween 30 mol % to 90 mol % of the mols of the glass, preferablybetween 50 mol % and 80 mol %. The higher the SiO₂ content in theglasses the more viscous they get and the easier they can be formed intolarge blocks. However, the fluoride retention is less than in glasseswhich have a SiO₂ content towards the lower limit. The SiO₂ can bereplaced e.g. by GeO₂ and Al₂O₃ by Ga₂O₃. The alkali content (Na₂O) canbe replaced fully or partly by other alkalis, mixture of alkalis oralkaline-earths such as BaO. Many other ingredients can be added to theNAS glass in order to modify and tailor the refractive index, expansion,durability, density and color of the glass matrix.

Preferably the crystal phase in the oxyfluorides comprise LaF₃.LaF₃-glass ceramics can be manufactured by heat treating (tempering)Al₂O₃ rich NAS glass saturated with LaF₃. The solubility of LaF₃ isdetermined by the Al₂O₃ in the glass. LaF₃ levels far below thesolubility limit results in stable glasses that do not form glassceramics when heat treated. Therefore the content of LaF₃ in the glasshas to be within ±15%, preferably 10% of the solubility limit of LaF₃.In case the alkali content is replaced by alkaline-earth compositionsthe solubility of LaF₃ is raised. Therefore the amount of LaF₃ should beincreased. LaF₃ glass ceramics shows a chemical resistance which is inmany aspects better than glass ceramics used before, e.g. ZBLAN glassceramics.

The LaF₃ crystal phase allows the partition of any rare earth. Thereforea huge variety of up- and down converting materials with very unusualelectronic structures can be provided, which are responsive toexcitation radiation not commonly used in product security. Thus thoseglass ceramics in combination with at least a two photon excitationaccording to the advanced product security system of the presentinvention broadens the application of up-converters substantially.

In a preferred embodiment the oxyfluoride glass ceramics is transparentand colorless to the human eye.

By controlling the correct microstructure, transparency of oxyfluorideglass ceramics may be achieved which is equivalent to best opticalglasses. Generally the microstructure of the LaF₃ glass ceramics is afunction of the heat treatment temperature. When heat treated at 750° C.for 4 h a large number of relatively small (ca. 7 nm) LaF₃ crystals arevisible. At higher temperature the crystallites grow larger. At 800° C.the average crystal has a dimension of 20 nm and at 825° C. over 30 nmaverage crystallite size is observed. Since appropriate crystallite sizeis the main influence factor for transparency, the glass ceramics formedat 750° C. for 4 h resulted in the most transparent of all. Even withthe increase of crystallite size related with the heat treatment up to775° C. the transparency was still higher than of untreated material.The transparency is measured as a function of the extinction which isthe sum of the total loss of scattering and absorption effects. Above850° C. the oxyfluoride glass ceramics becomes opaque.

The tempered glass ceramic can be ground to pigment. Optimal particlesize for most printing applications is in the order of 3 to 10 μm. Afterincorporating such transparent oxyfluoride glass ceramic particles intoa transparent coating or ink vehicle, an invisible product coding can beapplied to a substrate. Since the oxyfluoride glass ceramic pigments canbe designed with emission properties which do not respond to theexcitation radiation of commonly used wavelengths it becomes verydifficult for a potential counterfeiter to localise and identify themarking or to retro-engineer the pigment.

The coating composition, preferably printing ink, of the presentinvention further comprises binders. The binders used in the presentinvention may be selected from any of the polymers known in the art.Polymers useful in coating composition, preferably printing inks includealkyds, polyurethanes, acrylics, polyvinylalkohols, epoxy-resins,polycarbonates, polyesters, etc. The polymers may be thermoplastic,oxidatively crosslinkable or radiation curable e.g. under UV-radiation.In the latter cases the resins comprise suitable cross-linkablefunctional groups. Such groups can be hydroxy, isocyanate, amine, epoxy,unsaturated C—C bonds, etc. These groups may be masked or blocked insuch a way so that they are unblocked and available for thecross-linking reaction under the desired curing conditions, generallyelevated temperatures.

The above-described polymers can be self-crosslinkable or the coatingcomposition can include a separate cross-linking agent that is reactivewith the functional groups of the polymer.

The coating composition, preferably printing ink, of the presentinvention can be solvent- or water-borne. Although the printing ink orcoating composition of the present invention may be utilized in the formof substantially solid powder or dispersion a rather liquid state ispreferred. The organic solvents can be of the polar or apolar typedepending on the binder polymers employed.

Other pigments and or fillers may be present. The term “filler” isdefined according to DIN 55943:1993-11 and DIN EN 971-1:1996-09. Filleris a substance in granular or pulvery form which is insoluble in theother components of the coating composition, preferably printing ink andis used to provide and influence certain physical properties of theoverall composition.

The term “pigment” is to be understood according to the definition givenby DIN 55943:1993-11 and DIN EN 971-1:1996-09. Pigments are colouringmaterials in pulvery or plate-like dimensions which are—contrary todyes—not soluble in the surrounding medium. Functional pigments such asmagnetic, corrosion inhibiting- and/or electroconductive pigments may beemployed as well.

The coating composition, preferably printing ink may comprise otheradditives, such as rheology control agents, waxes, passive resins, i.e.resins which do not contribute to the film forming process, surfactants,soluble dyes, synergists, photoinitiators, etc.

The coating composition, preferably printing ink, may be applied to theunderlying substrate by any of the known deposition processes, such asspraying, brushing, dipping. Preferably it is applied by printingtechniques such as flexo-, gravure-, screen, intaglio, letter press andoffset printing.

What is claimed is:
 1. A coating composition for security applications,said coating composition comprising at least one organic resin and atleast one pigment wherein said pigment comprises glass ceramic compositeparticles which contain at least one crystalline phase embedded in aglass matrix, said pigment having a particle size between 0.1 μm and 50μm.
 2. A coating composition according to claim 1 wherein the pigmenthas a particle size between 1 μm and 20 μm.
 3. A coating compositionaccording to claim 1 wherein the pigment has a particle size between 3μm and 10 μm.
 4. A coating composition according to claim 1 wherein thecrystalline phase of the glass ceramic composite particle comprises aluminescent material.
 5. A coating composition according to claim 1wherein the crystalline phase of said glass ceramic composite particlehas a phonon energy not exceeding 580 cm⁻.
 6. A coating compositionaccording to claim 1 wherein the crystalline phase of said glass ceramiccomposite particle has a phonon energy not exceeding 400 cm⁻.
 7. Acoating composition according to claim 1 wherein the crystalline phaseof said glass ceramic composite particle has a phonon energy notexceeding 350 cm⁻.
 8. A coating composition according to claim 1 whereinthe glass ceramic composite particle is transparent to electromagneticradiation in the range of 400 to 750 nm.
 9. A coating compositionaccording to claim 1 wherein the crystalline phase of said glass ceramiccomposite particle has average dimensions not exceeding 50 nm.
 10. Acoating composition according to claim 1 wherein the crystalline phaseof said glass ceramic composite particle has average dimensions notexceeding 40 nm.
 11. A coating composition according to claim 1 whereinsaid crystalline phase of said glass ceramic composite particle containsat least one active ion for providing long wave to short wave lightconverting properties.
 12. A coating composition according to claim 1wherein said crystalline phase of said glass ceramic composite containsat least one active ion for providing short wave to long wave lightconverting properties.
 13. A coating composition according to claim 12wherein said active ion is a rare-earth ion.
 14. The coating compositionof claim 13 wherein the rare-earth ion is selected from the groupconsisting of Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺ andYb³⁺.
 15. A coating composition according to claim 1 wherein the glassceramic composite particle is an oxyfluoride glass ceramic.
 16. Acoating composition according to 15 wherein the crystalline component ofthe glass ceramic composite particle comprises LaF₃.
 17. A coatingcomposition according to claim 15 wherein the glass matrix consistsessentially of Na₂.OAl₂O₃.SiO₂.
 18. A security document having at leastone layer imprinted with a coating composition comprising at least oneorganic resin and at least one pigment, wherein the pigment comprisesglass ceramic composite particles containing at least one crystallinephase embedded in a glass matrix, said pigment having a particle sizebetween 0.1 μm and 50 μm.
 19. A pigment comprising at least one glassceramic composite particle material having luminescent properties.
 20. Amethod of producing a coating composition comprising glass ceramiccomposite particles as pigments, said method comprising steps ofproviding glass ceramic pigment by comminuting said glass ceramiccomposite material to a predetermined particle size, and incorporatingsaid glass ceramic pigment into a coating composition comprising atleast one organic resin and at least one pigment.